1. Field of the Invention
The present invention relates to a process for separating bitumen from tar sands and, in particular, to a process for separating relatively high viscosity bitumen from tar sands.
2. Description of the Background
Tar sand is a mixture of bitumen (tar) and sand. The bitumen content varies from deposit to deposit as well as within a given deposit. Tar sand deposits occur throughout the world, frequently in the same geographical area as conventional petroleum deposits. It has been estimated that about two-thirds of all of the known oil in the world is contained in tar sand deposits or in heavy oil deposits. In particular, large tar sand deposits have been identified and mapped in Canada, Venezuela and the United States.
Canadian tar sands represent one of the largest deposits in the world, having an estimated recoverable potential of approximately 900 million barrels. In the United States, although tar sand deposits have been reported in at least twenty two states, the largest and most significant deposits are found primarily eleven states: Alabama, Alaska, California, Kansas, Kentucky, Missouri, New Mexico, Oklahoma, Texas, Utah and Wyoming.
In Alabama, the most significant asphalt or tar sand deposits occur in the Pride Mountain formation and overlying Hartselle Sandstone. Past commercial development has been restricted largely to the provision of material for road construction and has been conducted primarily in Pride Mountain deposits in west-central Colbert County. The widespread Hartselle Sandstone, however, contains the largest reserves of petroleum and these rocks are considered to have the best potential for future oil-extraction operations. The bitumens in these rocks is characterized primarily as a typical asphalt containing hydrocarbons that have been variably biodegraded.
Further, core analyses show that the Hartselle Sandstone has widely variable porosities, permeabilities and oil saturations. Porosities of bitumen--impregnated intervals range from less than 5% to as much as 24%, and permeabilities range from less than 10 millidarcies to hundreds of millidarcies. Oil saturations range from only a trace to zones containing in excess of 700 barrels per acre-foot. Some localized areas contain in excess of 12,000 barrels of bitumen per acre. Reserves in the outcrop area are estimated to be approximately 350 million barrels at depths shallower than 50 feet, and some 7.5 billion barrels are thought to be present in the subsurface within an area of fifty square miles.
Throughout the past fifty years, various processing strategies for tar sands have been tested. Due to the significant differences in the physical and chemical nature of Utah or Alabama tar sands as compared to Canadian tar sands and because of the great differences in climatic conditions between the two locations, however, the technology developed for Canadian tar sands cannot be directly applied to Utah tar sands. An example of a process that has been developed specifically for Utah or Alabama tar sands is set forth in U.S. Pat. No. 4,120,776. This process is, in essence, a hot water process wherein a hot, aqueous solution having a controlled pH range is used to displace the bitumen from the sand.
While relatively good separation of the bitumen from the tar sand has been obtained using variations in the hot water separation process, any hot water processing methodology necessarily requires substantial energy input. For example, it has been estimated that the required energy input for digestion in the hot water process, operating at 95.degree. C. and obtaining about 90% bitumen recovery, requires at least 45 kilowatt hours of energy per ton of tar sand processed.
More recently, in an attempt to reduce energy consumption and to recover bitumen from tar sands at ambient temperature, Miller et al, U.S. Pat. No. 4,486,294, disclose a process for recovering bitumen from tar sand, which entails, in essence: grinding the tar sand to mechanically fracture the bitumen and thereby disengage bitumen particles from sand particles, mixing a wetting agent with the tar sand during grinding to disengage bitumen particles from the sand particles, conditioning the bitumen particles with an oil to enhance the hydrophobicity of the bitumen particles, and separating the bitumen particles from the sand particles by flotation. U.S. Pat. No. 4,486,294 uses conventional grinding techniques.
However, while a reduced energy consumption is maintained with the above method, grinding or comminution achieves only limited success in effecting a phase disengagement in tar sands. Further, it has been determined that if grinding or comminution is carried on for too long a time the process of disengagement is actually reversed and the quality of separation is reduced.
Thus, a need exists for a process for separating bitumen from tar sands reduced energy requirement, while obtaining a good yield of bitumen in effecting separation.